1. Field of the Invention
The present invention relates to a process for the preparation of an isocyanate, particularly a process for preparation of an aliphatic isocyanate by phosgenating a aliphatic polyamine or hydrochloride thereof.
The isocyanate obtained by the invention is a very useful compound as a raw material for polyurethane-based materials, polyurea-based materials and polyisocyaurate based materials in chemical, resin and paint industries.
2. Description of the Prior Art
Several processes for preparing aliphatic isocyanate by phosgenating aliphatic amine or a salt thereof are known; and the following processes have been proposed.
(1) A process for carrying out phosgenation of aliphatic amine or its hydrochloride at temperature of 120.degree. to 180.degree. C. in a weight ratio of solvent/raw material amine of 18/1 to 30/1 [British Patent 1,086,782].
(2) A process for reacting phosgene with a mixture of an aliphatic triamino compound and hexamethylenetriamine having a ratio of 95:5 to 90:10 by weight [Japanese Patent Laid-Open Publication SHO 60-233,044(1985) ].
Other processes which have been proposed include continuous high-temperature phosgenation processes [Japanese Patent Laid-Open Publication SHO 55-88,451(1980) ], increased pressure phosgenation processes [U.S.P 2,642,449] and vapor phase phosgenation processes [Japanese Patent Laid-Open Publication SHO 63-280,050(1988)].
Processes to prepare isocyanates by reacting primary amines with phosgene in an inert solvent have been known. When the primary amines are an aromatic amine, the aromatic amine can be converted with comparative ease to a high-purity aromatic isocyanate by passing phosgene gas through a suspension of free base or hydrochloride of the aromatic amine in the solvent. In the case of aliphatic amines, however, the reaction with phosgene is generally slow compared with the reaction of an aromatic amine with phosgene, and forms, as is well known in the art, chloroderivatives as by-products due to a deamination reaction.
The chlorinated impurity is usually formed in an amount of 3 to 10% by weight and can sometimes be has high as 20% by weight. Hence the yield of the desired product undergoes a corresponding decrease.
Formation of the chlorinated impurity is primarily observed in the syntheses of aliphatic isocyanates, and are typically not found in the preparation of aromatic isocyanates.
When aliphatic isocyanates containing the chlorinated impurity are used for a polyurethane-based material, the chlorinated impurity affects the reaction of the isocyanate group with active hydrogen containing compounds. That is, the chlorinated impurity inhibits the reaction, accelerates gelation of the prepolymer and further exerts an adverse effect on the properties of the resulting polyurethane resin.
No difference is generally observed between the properties of the chlorinated impurity and corresponding isocyanate except that the boiling point of the chlorinated impurity is generally from 5.degree. to 20.degree. C. lower than that of the corresponding isocyanate. Specific procedures are hence required for removing the impurity and separating a high-purity aliphatic isocyanate.
Consequently, the above-mentioned processes have been proposed in order to reduce the formation of impurity as much as possible in the phosgenation step. These processes of the prior art, however, have been disadvantageous in that use of a large amount of solvent leads to low volume efficiency and very poor economy, and that extensive purification equipment is required for separating the impurity from the desired isocyanate. As a result, conventional processes have been unsatisfactory in terms of industrial production.